The present invention relates to aberration correction for coherent imaging systems with sampled apertures, in particular, to correction of aberration caused by tissue inhomogeneities in medical ultrasound.
Tissue inhomogeneities distort the wavefront delay, amplitude and spectral characteristics. This causes an increase in clutter and reduction in detail resolution and signal-to-noise ratio (SNR). By far the most dominant source of tissue aberration is the delay aberration. Various methods have been described for delay aberration estimation. The methods are based on transmit beams focused at the region of interest. The receive element signals received in response to a focused transmit beam are cross-correlated to estimate delay aberration. The estimated delay aberration is then used to modify the transmit and receive delay profiles for aberration correction. As an alternative to having a correlation-based estimation stage, the receive delay profiles are perturbed systematically until an image quality measure such as the root mean square amplitude of the beamformer output is maximized, such as disclosed in U.S. Pat. No. 6,368,279.
Higher correlation of the receive element signals provides better aberration estimation. If the object is incoherent (i.e., speckle generating) as most tissues are, the correlation of the receive element signals increases with better transmit focus (the van Cittert-Zernike Theorem). Therefore aberration estimation and correction steps may need to be iterated over multiple transmit events, improving the transmit focus and hence the estimation and correction at each iteration.
In cases where delay aberration can be modeled as a phase screen at a distance away from the transducer surface, the receive element signals are back propagated to the phase screen depth to increase correlation of element signals before the cross correlation. The back propagation methods: the diffraction integral method, the angular spectrum method and the shift-and-add method are described in D-L Liu and R. Waag, “Propagation and Back propagation for Ultrasonic Wavefront Design”, IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 44, no 1, January 97.
Where aberrations are distributed in space and not limited to a phase screen at the surface of the transducer, there may be more than one isoplanatic patch in the region of interest. Then, the aberration estimation and correction are repeated using additional transmit and receive events for each isoplanatic patch. Therefore, aberration correction is more time consuming for smaller isoplanatic patch sizes. Providing for more frequent or a greater number of transmit events reduces imaging frame rate.
Aberration estimation techniques that rely on focused transmit beams are not convenient to use in conjunction with imaging techniques that rely on broad transmit beams. One broad transmit beam technique is to form many parallel receive beams in response to the broad transmit beam (receive multibeam). Another broad transmit beam technique is the synthetic transmit aperture technique, such as disclosed in U.S. Pat. No. 6,551,246. The transmit aperture is synthesized by coherently summing a set of images, each formed in response to a broad transmit beam using receive multibeam. A broad transmit beam is formed typically by exciting either a single (real) transmit element, or a set of transmit elements delayed to mimic a virtual transmit element. The receive multibeam and synthetic transmit aperture techniques provide high frame rate imaging, especially for 3D. However, due to reduced degree of data redundancy, images formed with broad transmit beams are more susceptible to aberration effects. Therefore there is a need for aberration correction techniques that are convenient to use with broad transmit beams.